A slice of semiconductor material is machined, for example, to obtain integrated circuits or other electric components in the semiconductor material. In particular, when the slice of semiconductor material is very thin, the slice of semiconductor material is placed on a support layer (typically made of plastic material or glass) that serves to provide a higher mechanical sturdiness and thus an ease in handling.
In general it is necessary to mechanically machine the slice of semiconductor material by grinding and polishing to obtain a thickness that is uniform and equal to a desired value. In this phase of the mechanical machining of the slice of semiconductor material the thickness needs to be measured for ensuring that the desired value is accurately obtained.
For measuring the thickness of a slice of semiconductor material it is known to use gauging heads with mechanical feelers touching an upper surface of the slice of semiconductor material being machined.
This measuring technology can cause damage to the slice of semiconductor material during the measurement owing to the mechanical contact with the mechanical feelers, and does not enable to measure very thin thicknesses (typically lower than 100 micron).
For measuring the thickness of a slice of semiconductor material it is known to use capacitive probes, inductive probes (eddy current probes or other types), or ultrasound probes. These measuring technologies are of the contactless type and thus do not damage the slice of semiconductor material during the measurement and can measure the thickness of the slice of semiconductor material even when there is the support layer. However, these technologies are limited both in the dimensions they can measure and in the highest resolution they can achieve.
For overcoming the limits of the measuring technologies above described, optical probes and interferometric measurements are used. For example, the international patent application published with No. WO2009013231A1, the U.S. Pat. No. 6,437,868A1 and the Japanese patent application published with No. JP082160016A describe apparatuses for optically measuring the thickness of a slice of semiconductor material.
Some of the known apparatuses include a source of light radiations, that are mostly infrared radiations as the currently used semiconductor materials are silicon based and the silicon is sufficiently transparent to the infrared radiations, or have a wider spectrum for enabling the measurement of particularly thin thicknesses. The emitted radiation beam features a low coherence and a plurality of wavelengths within a determined band. Such apparatuses further include a spectrometer and an optical probe that is connected to the source of light radiations and to the spectrometer by means of optical fibres, faces the slice of semiconductor material to be measured, and is provided with lenses for focusing the radiations emitted by the radiation source on the slice of semiconductor material to be measured and for collecting the radiations reflected by the slice of semiconductor material to be measured. A spectral analysis of the combinations resulting from the interference of the radiations that are reflected by the external surface and by possible optical discontinuity surfaces inside the slice of semiconductor material to be measured is carried out by means of the spectrometer. From such a spectral analysis of the combinations resulting from the interference of the radiations reflected by the slice of semiconductor material it is possible to determine the measure of the thickness of one or more layers of optically homogeneous material that have been crossed by the radiations.
But the above-mentioned analysis does not enable to determine the path followed by the reflected radiations that are combined. In other words, the combinations are the end result of a plurality of reflections occurring on the external surface of the slice of semiconductor material and inside the latter at each optical discontinuity surface. But in the known apparatuses it is not possible to use information that could be present in the combinations of reflected radiations to directly or indirectly measure the distance between the optical probe and each of the discontinuity surfaces causing the reflections. As a consequence, the analysis of the combinations of radiations reflected by the slice of semiconductor material enables to determine the measure of the thickness of the layers placed between couples of optical discontinuity surfaces, but it is not possible to determine the part of the slice of semiconductor material to which the measure of the thickness has to be assigned (that is to determine whether the measure of the thickness has to be assigned to a first layer that has been crossed twice, to the first layer that has been crossed n times, to a second or third layer, or to the first layer added to the second layer, etc.).
At each reading it is not only a single radiation reflected by the slice of semiconductor material that is analyzed but a beam of radiations reflected by the slice of semiconductor material. Therefore, the measures of a plurality of different thicknesses are determined, but it is not possible to assign each measure of thickness to a specific part or layer of the slice of semiconductor material. However, for each reading it is possible to determine a corresponding quality factor on the basis, for example, of the ratio between the specific luminous power and the overall luminous power. Indeed, the quality factor is one of the clues suggesting that the associated reading corresponds to the thickness to be measured.
A known apparatus for optically measuring by interferometry the thickness of a slice of semiconductor material provides at each reading rough thickness values and associated quality factors on the basis of which they are generally arranged. In order to succeed in identifying, among all the rough thickness values provided by the apparatus, which rough thickness values correspond to the first layer of the slice of semiconductor material (i.e. the most external layer that is made of semiconductor material, is subject to grinding of polishing, and the thickness of which is to be measured) the known apparatuses use a recognition algorithm analyzing a relatively high number of consecutive readings (typically at least some tens of consecutive readings). Such known recognition algorithm considers, for each reading, only the rough thickness value with the highest quality factor. Then, all the rough thickness values with a quality factor lower than a minimum quality threshold and all the rough thickness values being lower than a minimum reject threshold or higher than a maximum reject threshold —the reject thresholds define the range within which the wanted thickness value has to lie—are rejected. Finally, the wanted thickness value (that is the measure of the thickness of the most external layer made of semiconductor material) can be determined by averaging the remaining rough thickness values.
However, some known recognition algorithms, as the one described above, have several inconveniences.
First of all, the accuracy of the known recognition algorithm described above is not optimal and extremely variable over time: the recognition algorithm is accurate when there are no foreign thickness values that are similar to the wanted thickness value, but it is much less accurate when there are foreign thickness values that are similar to the wanted thickness value.
Moreover, to obtain an acceptable accuracy, a special attention has to be paid in choosing both the minimum quality threshold and the reject thresholds serving to cut the thickness values provided by the readings. In other words, there are no minimum quality thresholds and reject thresholds that apply to all situations, but every time it is necessary to adapt the minimum quality thresholds and the reject thresholds to the specific current situation. Hence, the choice of the minimum quality thresholds and the reject thresholds requires every time the intervention of an experienced operator who is able to analyze the rough thickness values provided by the readings beforehand.
The intervention of an experienced operator is normal and thus acceptable in laboratory measurements but is not possible in measurements carried out in a production line during the serial production.